All we could do was wait.
Luckily the coffee I brewed for open was still hot so we had some caffeine, and we broke open a couple pastries to sample while sitting at the window and telling unobservant customers who walked up to the door without realizing that no lights means no coffee that we're temporarily closed.
After an hour and a couple calls in to Ameren to get a potential power up time (to no avail), I called our store manager to see what to do. She told us we had to stay until it came back on and just find stuff to do in between monitoring fridge temps. We moved all the sandwiches and retail food to one of the back freezers (I figured it would take longer for those fridges to lose their cool), and that was pretty much all we could do in the darkness so "stuff to do" became chatting, reading the newspaper, and sampling another pastry.
After three hours, the lights, music, and heater came back on and within ten minutes we had a line to the door. Back to a typical Sunday morning at Starbucks!
(photo by e.hunt)